Enhanced Intel SpeedStep Technology (EIST) is a
power and thermal management technology
developed by Intel. EIST was introduced as a
means of enabling high performance while
meeting the power-saving needs of a mobile
computer system.

Essentially, EIST throttles the clock speed of
a central processing unit (CPU) during periods
requiring minimal demand. It then brings the
clock speed back up to its maximum potential
when demanded by the load . This allows a
computer to save power when there is less to
process, yet still achieve high performance
when the demand is high.

The earlier, non-enhanced version of SpeedStep
switched frequency and voltage between low and
high levels in response to a current processor
load. EIST builds on this by using the
following strategies:

Separating frequency and voltage
changes, whereby voltage is stepped up or down
in small increments separately from changes in
frequency. Because of this, the processor is
able to reduce system unavailability due to
frequency change. This technique allows the
system to switch between voltage and frequency
states more often, improving power-performance
balance.

Clock partitioning and recovery, where the
bus clock runs continuously even during state
transitions. It continues running even when the
core clock and the phase-lock loop are stopped.
This allows for logic to remain active even
when some of the CPU portions are currently
stopped.

EIST reduces the latency inherent with
changing
the voltage-frequency pair (P-state), thus
allowing those transitions to occur more
frequently. This allows for more granular,
demand-based switching and can optimize the
power-to-performance balance, based on the
demands of the applications.